Brushless DC motors are in widespread use. A brushless DC motor employs a permanently magnetized rotor and electronic commutation to switch current to appropriate stator windings to cause the rotor to rotate to follow switched magnetic poles in the stator windings. Brushless DC motors may be commutated by signals generated from the rotational velocity and position of the rotor to cause the appropriate stator winding to be switched to sustain rotation. Furthermore, brushless DC motors may be commutated by an external frequency source to cause the rotor to rotate at a rotational velocity synchronous with the external signal source. Mechanisms for detecting the rotational velocity and position of the rotor include resolver windings, Hall effect devices, optical position sensors, etc.
When a brushless DC motor is accelerated from rest to a high velocity such as 70,000 rpm in an application such as driving a compressor, several mechanical resonance points are encountered. During acceleration of a brushless DC motor through mechanical resonance points, synchronous operation is difficult to sustain for the reason that if the mechanical resonance is severe the rotor can lose synchronous speed torque which will result in the motor stalling. Accordingly, in order to rapidly accelerate a brushless DC motor up to a high speed of the type used for driving compressors such as 70,000 rpm, it is highly desirable to rapidly accelerate the rotor through the mechanical resonance points by utilizing a commutating signal that is derived from the position and rotational velocity of the rotor which is not subject to stalling as is the case when the motor is commutated in a synchronous mode of operation.
U.S. Pat. No. 3,706,923 discloses a brushless DC motor acceleration system in which the rotor is accelerated with electronic commutation derived from the velocity and position of the rotor until a predetermined velocity is obtained at which point commutation is switched over to an external signal source. The motor switches from a closed loop feedback circuit to an open loop operation upon sustaining the synchronous speed. No provision is made in the '923 patent for switching the motor from internally generated electronic commutation which is controlled by the rotational velocity and position of the rotor to commutation controlled by an external signal source precisely when the internally generated pulses for switching the windings and the externally generated pulses for switching the windings are time coincident. Accordingly, the '923 patent has no mechanism for precisely controlling the crossover point between internally generated and externally generated commutation to insure that there is a minimum discontinuity in frequency and phase at the instant of transfer.
U.S. Pat. No. 3,979,651 discloses a motor operable in a direct current and synchronous mode. The '651 patent discloses a system for accelerating the rotor under the control of a commutation signal generated from the velocity and position of the rotor up to a point where the rotor reaches a first velocity after which the commutation signal is externally generated. An up/down counter having inputs respectively responsive to a reference clock and from a pulse source having a frequency directly proportional to the rotational velocity of the rotor is used to detect the point at which control of electronic commutation is switched from a signal derived from the rotation and position of the rotor to an external reference signal. The '651 patent does not disclose any mechanism for precisely controlling the point at which control of electronic commutation is transferred from a signal generated from the velocity and position of the rotor to an external signal source to produce a minimum discontinuity in frequency and phase at the instant of transfer.
U.S. Pat. No. 4,353,016 discloses a linear motor control system for a brushless DC motor having an acceleration circuit which utilizes pulses generated from the velocity and position of the rotor to control electronic commutation of the windings of the motor until a predetermined speed is reached at which point control of commutation of the motor windings is transferred to an external frequency source. The '016 patent does not disclose any mechanism for precisely controlling the point at which control of electronic commutation is transferred from a signal generated from the velocity and position of the rotor to an externally generated signal for controlling electronic commutation with a minimum of discontinuity in frequency and phase at the instant of transfer.